Multilayered tablets, including trilayer tablets, have been used to modulate the release of an active compound, to increase absorption and bioavailability of an active compound, to deliver fixed-drug combinations, to provide a physical barrier between incompatible substances, and to mask unpleasant taste.
U.S. Pat. No. 5,342,627 discloses a device for the release of at least one active substance into a fluid medium by dissolution of the active substance in the medium, wherein the device comprises a covering that is impermeable to the active substance and the fluid medium and which has at least one aperture defining a shaped cavity filled by a core containing the active substance. The geometry of the device is selected to maintain a constant surface area of exposed active substance for a significant period to achieve a constant release of active substance over that period.
U.S. Pat. No. 6,960,357 discloses a controlled release dissolution and diffusion device which can deliver an active ingredient at a constant or controlled-variable rate. The device comprises an active ingredient and dissolution modifiers and/or an insoluble matrix.
The matrix is coated, except for at least one exposed face, with a coating containing an insoluble polymer or a mixture of an insoluble polymer and pore-forming elements. The delivery device is said to produce linear (zero-order) kinetics for releasing a chemical compound over a desired period. The disintegration rate of the coating can be manipulated by changing the concentration and/or size of the pore-forming materials, the water solubility of the materials, and/or the thickness and composition of the coating.
U.S. Pat. No. 5,853,760 discloses a device for controlling the release of an active substance in a liquid media from an active substance-containing matrix. The device includes exposed matrix surfaces for releasing the active substance, and surfaces that are covered by an erodible mass of solids. The use of thickness gradients is said to provide an erodible mass that is eroded at a rate intended to effect an enlargement of the contact surface of the active substance-containing matrix.